1. Field of the Invention
This invention relates generally to lightweight structures for spacecraft and other applications and more particularly to a novel wire mesh structure of this kind and to its method of fabrication.
2. Prior Art
As will appear from the later description, the wire mesh structure of the invention may assume a variety of forms depending upon its intended use. The particular structure described is an antenna reflector, specifically a parabolic reflector, for spacecraft.
A high premium is placed on the weight of spacecraft components, such as antennas, which must be traded off against all aspects of the total system performance to obtain an optimum design. This is particularly true in systems requiring parabolic surfaces as antenna reflectors. The technical requirements of minimum distortions for high performance, a need for stowage during the boost phase, combined with exposure to the extremes of the orbital environment after deployment, place severe constraints on the design.
A variety of spacecraft antennas have been devised in an attempt to satisfy the above and other design constraints. These existing antennas, however, fail to fully satisfy all the constraints. For example, antennas having rigid reflector surfaces, such as utilized in the sunflower concept. have the disadvantage of relatively high weight ratios and stowage space difficulty; coated fabric surfaces have the disadvantage of poor electrical characteristics and deterioration in the space radiation environment; and metallic fabric surfaces, in general, have the disadvantage of extreme sensitivity to dimensional tolerances, and are subject to large temperature excursions which cause large thermal distortions and thus result in significant areas of slack mesh between supports.
The earlier mentioned copending applications describe a wire mesh antenna reflector and its method of fabriction which avoid the above noted and other disadvantages of the existing reflectors and satisfy the stated spacecraft antenna design constraints. Simply stated, the antenna reflector is characterized by a wire mesh reflecting surface having wires which constitute primary structural elements of the mesh and are attached at their ends to the reflector frame. These structural elements or wires are preformed to a low rate spring-like configuration which renders the wires resiliently compliant in the endwise direction and are prestressed in a manner such that the wire mesh remains taut under widely varying thermal conditions, thus avoiding out-of-plane displacement of the mesh.
The particular antenna reflector described is a parabolic reflector having a supporting frame with a central reflector dish and ribs extending radially from the dish, flush with the front face thereof. These ribs and the front face of the reflector dish conform to a common parabolic surface. The spaces between adjacent ribs are spanned by wire mesh gores having wires, referred to herein as hoop wires, extending generally circumferentially or hoopwise of the reflector and other wires, referred to as radial wires, extending generally radially of the reflector. The hoop wires are terminally secured to the ribs and constitute the compliant structural wires of the mesh. The radial wires stabilize the mesh and cooperate with the hoop wires to provide the required electrical characteristics of the reflector.
According to the reflector gore fabricating method described in the copending applications, the gore hoop wires are first formed to their spring configuration and are then prestressed to a tension, referred to as a preload tension or simply a preload, greater than the maximum tension load exerted on the hoop wires in actual use of the antenna reflector in the space environment. This preload insures that the tension loads exerted on the hoop wires in use will not produce permanent deformation of these wires, which would alter their spring rate or stiffness, and permits the effective spring rate or stiffness of the reflector mesh to be accurately predetermined and maintained over the full service life of the antenna reflector. After preloading, the hoop wires are welded to radial wires to form wire mesh gores which are assembled on the antenna reflector frame to form a finished wire mesh antenna reflector surrounding the central reflector dish. The steps of welding of the hoop and radial wires to form wire mesh reflector gores and assemblying these gores on the reflector frame to form a finished wire mesh antenna reflector are performed in a manner which establishes in the hoop wires of the finished reflector a predetermined tension, referred to as an initial or assembly tension. This assembly tension is made sufficiently high to insure that the fluctuations in hoop wire tension occasioned by the changing thermal conditions to which the reflector is exposed in the space environment will not produce complete relaxation of the hoop wire tension, such that the reflector mesh would become slack. The reflector mesh thus remains taut, to retain the parabolic configuration of the reflector, over the entire range of thermal conditions encountered by the reflector in use.
The wire mesh structure of the copending applications is characterized by compliant spring wires of uniform spring rate or stiffness throughout the entire area of the wire mesh. Such a uniform spring rate is quite satisfactory for many wire mesh structures as, for example, those whose conditions of use are such that all of the mesh spring wires are subjected to substantially the same tension loads or stress fluctuations. On the other hand, the conditions of use of some wire mesh structures are such that the tautness of the wire mesh could not be maintained if all of the mesh spring wires had the same spring rate or stiffness. Examples of these latter use conditions are those which subject the spring wires in different areas of the wire mesh to substantially different thermally induced tension loads or stress fluctuations and those which subject some spring wires primarily to thermally induced tension loads and other spring wires to inertial loads.